Answer: The catalytic converter
is our main line of defense against air pollution, so it's important to
make sure it is functioning efficiently and passing exhaust without creating
undue restrictions that might reduce performance, fuel economy or emissions.
If the your converter is plugged, it will create a restriction in your
exhaust system. The buildup of backpressure will cause a drastic drop
in engine performance and fuel economy, and may even cause the engine
to stall after it starts if the blockage is severe.
easiest test for converter plugging is done with a vacuum gauge. Connect
the gauge to a source of intake vacuum on the intake manifold, carburetor
or throttle body. Note the reading at idle, then raise and hold engine
speed at 2,500. The needle will drop when you first open the throttle,
but should then rise and stabilize. If the vacuum reading starts to drop,
pressure may be backing up in the exhaust system.
can also try to measure backpressure directly. If your engine has air
injection, disconnect the check valve from the distribution manifold,
and connect a low pressure gauge. Or, remove the oxygen sensor and take
your reading at its hole in the manifold or headpipe. Refer to the backpressure
specs for the application. Generally speaking, more than 1.25 psi of backpressure
at idle, or more than 3 psi at 2,000 rpm tells you there's an exhaust
restriction. If there appears to be an exhaust restriction, disconnect
the exhaust pipe just aft of the converter to relieve pressure and recheck
the readings. Caution: The pipes will be hot so wait awhile for things
to cool down. If vacuum goes up and/or backpressure drops, the problem
isn't not a plugged converter but a plugged muffler or collapsed pipe.
If there's little or no change in readings, the converter is plugged.
because a converter is passing gas doesn't mean it is okay. If the catalyst
inside is contaminated or worn out, high carbon monoxide (CO) and/or hydrocarbon
(HC) readings will be present in the exhaust. If you have access to a
high temperature digital pyrometer (or an oven thermometer will do), check
the converter's temperature fore and aft. A good converter will usually
run several hundred degrees hotter at its outlet than its inlet. Little
or no temperature change would indicate low efficiency, or a problem with
the converter's air supply. Converters need supplemental oxygen in the
exhaust to reburn pollutants, so if the air injection system or aspirator
valve isn't doing its job the converter can't do its job either.
the air injection pump, belt and check valve. If you suspect that the
check valve is allowing exhaust to flow backwards, remove it and blow
through both ends. It should let air pass in one direction, but not in
the other. Examine the air injection manifold, too, because it tends to
rust out and leak air. Check the diverter valve to make sure it is working
correctly, too. It should be routing air to the converter when the engine
is at normal temperature. On engines with aspirator valves instead of
air pumps, you should hear and/or feel the fluttering of the internal
flapper as the engine is idling.
of converter failures: Fouling, clogging, melt-down and breakage of the
ceramic substrate inside a converter are common conditions that can cause
problems. Plugging is usually the end result of a melt-down, which occurs
because the converter gets too hot. This happens because the engine is
dumping unburned fuel into the exhaust. The excess fuel lights off inside
the converter and sends temperatures soaring. If it gets hot enough, the
ceramic substrate that carries the catalyst melts. The unburned fuel may
be getting into the exhaust because of a bad spark plug or valve, but
an overly rich air/fuel mixture is another possibility. In older carbureted
engines, a heavy or misadjusted carburetor float may be the underlying
cause. But on newer engines with "feedback" carburetion or electronic
fuel injection, the engine may not be going into "closed loop"
(the normal mode where the computer regulates the air/fuel mixture to
minimize emissions). A bad oxygen sensor or coolant sensor may be giving
the computer bogus information. A sluggish or dead O2 sensor will make
the computer think the exhaust is running lean, so the computer will try
to compensate by making the fuel mixture rich. A coolant sensor that always
indicates a cold engine will also keep the system in open loop, which
means a steady diet of excess fuel. But it might not be the sensor's fault.
A thermostat that's stuck open or is too cold for the application can
prevent the engine from reaching its normal operating temperature. So
if your converter has failed and needs to be replaced, the engine should
be diagnosed for any underlying problems before the new converter is installed.
Another cause of converter clogging and contamination is excessive oil
consumption. Worn valve guides or seals can allow oil to be sucked into
the engine's combustion chambers. The same goes for worn or damaged rings
or cylinders. Oil can form a great deal of carbon, and metals present
in the oil can contaminate the catalyst. A compression check or leak-down
test will tell you if the rings are leaking, while a fluttering vacuum
gauge needle will help you identify worn valve guides.
The muffler is an oval-shaped or cylindrical component made from
coated and aluminized steel or stainless steel. Inlet and outlet pipes
extend from the ends of the muffler. Inside the muffler, the exhaust gas
flows through a series of perforated tubes and a tuning chamber to silence
the exhaust. The perforated tubes inside the muffler cancel out and silence
the pressure pulsations in the exhaust each time an exhaust valve opens.
The muffler is located behind the catalytic converter in the exhaust system.
On many vehicles, the muffler is positioned just behind the center of
the vehicle, but space requirements on some vehicles demand muffler installation
near the rear of the car. When the muffler is positioned near the rear
of the vehicle, it runs cooler and may experience more internal condensation.
Mufflers rust on the inside if excessive internal condensation occurs.
The most common type of muffler is the reverse-flow design, which changes
the direction of exhaust flow inside the muffler. Some mufflers are a
straight-through design in which the exhaust passes through a single perforated
Mufflers: Although electronic mufflers are not widely used at present,
they may be installed on vehicles in the near future. In an electronic
muffler system, sensors and microphones in the exhaust system sense the
pattern of exhaust pressure waves. This information is sent to an on-board
computer that controls loudspeakers in the muffler. The computer operates
the loudspeakers to generate sound waves that oppose and cancel the original
exhaust sound waves produced by the engine.
The tail pipe carries the flow of exhaust from the muffler to the rear
of the vehicle. Some vehicles have an integral resonator in the tail pipe.
This resonator is similar to a small muffler, and it provides additional
exhaust silencing. In some exhaust systems, the resonator is clamped into
the tail pipe. Tail pipes have many different bends to fit around the
chassis and driveline components. All exhaust system components must be
positioned away from the chassis and driveline to prevent rattling. The
tail pipe usually extends under the rear bumper, and the end of this pipe
is cut at an angle to deflect the exhaust downward. Chrome tail pipe extensions
are available in auto parts stores. These extensions are attached to the
tail pipe with lock screws.